18 
DARWINISM 
CHAP. 
so as to keep out cattle. On ascertaining this, Mr. Darwin 
was so much surprised that he searched among the heather in 
the unenclosed parts, and there he found multitudes of little 
trees and seedlings which had been perpetually browsed down 
by the cattle. In one square yard, at a point about a hundred 
yards from one of the old clumps of firs, he counted thirty- 
two little trees, and one of them had twenty-six rings of 
growth, showing that it had for many years tried to raise its 
head above the stems of the heather and had failed. Yet 
this heath was very extensive and very barren, and, as Mr. 
Darwin remarks, no one would ever have imagined that cattle 
would have so closely and so effectually searched it for food. 
In the case of animals, the competition and struggle are 
more obvious. The vegetation of a given district can only 
support a certain number of animals, and the different kinds 
of plant-eaters will compete together for it. They will also 
have insects for their competitors, and these insects will be 
kept down by birds, which will thus assist the mammalia. 
But there will also be carnivora destroying the herbivora ; 
while small rodents, like the lemming and some of the field- 
mice, often destroy so much vegetation as materially to affect 
the food of all the other groups of animals. Droughts, Hoods, 
severe winters, storms and hurricanes will injure these in 
various degrees, but no one species can be diminished in 
numbers without the effect being felt in various complex ways 
by all the rest. A few illustrations of this reciprocal action 
must be given. 
Illustrative Cases of the Struggle for Life. 
Sir Charles Lyell observes that if, by the attacks of seals 
or other marine foes, salmon are reduced in numbers, the 
consequence will be that otters, living far inland, will be 
deprived of food and will then destroy many young birds or 
quadrupeds, so that the increase of a marine animal may 
cause the destruction of many land animals hundreds of miles 
away. Mr. Darwin carefully observed the effects produced 
by planting a few hundred acres of Scotch fir, in Staffordshire, 
on part of a very extensive heath which had never been 
cultivated. After the planted portion was about twenty-five 
years old he observed that the change in the native vegetation 
